Plecos are very territorial towards others of the same species. Do not keep two Plecos in one tank or you will have issues. It may work out right away but keep a close eye on them at all times. Their attacks are very abrupt and sudden.
As far as other fish species you have a very large spectrum of choices. Other fish have a tendency to leave them alone just because they said on the bottom and don’t really do anything except feed. To mention that their skin is known as an armor in the fish species habitat. It really doesn’t seem like a fish that you would want to fight with (to other fish). The stories you do read one Fish actually come after them and set up in a bad scenario. The top dorsal fin of the Pleco is very pointy and fish will have great deal of issues trying to eat it.
The only fish that we hear of having issues with the Pleco are unusually aggressive fish. Species that come to mind quickly might be.. Piranhas, Oscars and such. Some fish you might want to stay clear of our very flat fish to where the bottom feeder can get it sucker stuck on the side of fish and possibly kill it. You have to remember that this fish love sucking on things and will do so all day long!